An airbag cover of an airbag system for an automobile is demanded to have appropriate stiffness, securely burst in case of collision so that the airbag inflates and prevent broken pieces from scattering, have strength to allow its use even in cold regions and have heat resistance. As the material used for the airbag cover, there are proposed, for example, a composition containing a random polypropylene, a low density polyethylene and an ethylene copolymer rubber or a material comprised of a composition containing an olefin resin and an olefin rubber (Japanese Patent Laid-Open No. 8-27331, Japanese Patent Laid-Open No. 10-273001 and Japanese Patent Laid-Open No. 10-265628). Conventionally, an airbag cover has been formed by injection molding using such a material.
In addition, in some cases, an airbag apparatus for a front passenger seat is mounted on an instrument panel which is an interior part of an automobile. The airbag apparatus is comprised of an airbag which expands when the automobile collides with something and absorbs the impact to the human body, an inflator (gas generator) for expanding the airbag, an airbag cover for accommodating the airbag and the like. Generally, since the instrument panel is required to have heat resistance, it is comprised of a material composed mainly of a polypropylene resin. On the other hand, the airbag cover is required to have excellent mechanical properties such as low-temperature impact resistance, shape retention properties and workability when modularizing with other airbag parts, and as a material satisfying the required performance, an olefin thermoplastic elastomer composition is used (Japanese Patent Laid-Open No. 7-53828 and Japanese Patent Laid-Open No. 8-27331). As a method for integrating the instrument panel and the airbag cover, there is known a method for joining the back side (inside) of the instrument panel and the airbag cover by vibration welding (Japanese Patent Laid-Open No. 2004-98734 and Japanese Patent Laid-Open No. 2004-231027).